Polyesters have been replacing glass and metal packaging materials due to their lighter weight, decreased breakage compared to glass, and potentially lower cost. One major deficiency with standard polyesters, however, is its relatively high gas permeability. This curtails the shelf life of carbonated soft drinks and oxygen sensitive beverages or foodstuff such as beer, wine, tea, fruit juice, ketchup, cheese and the like. Organic oxygen scavenging materials have been developed partly in response to the food industry's goal of having longer shelf-life for packaged food. These oxygen scavenging materials are incorporated into at least a portion of the package and remove oxygen from the enclosed package volume which surrounds the product or which may leak into the package, thereby inhibiting spoilage and prolonging freshness.
Suitable oxygen scavenging materials include oxidizable organic polymers which may react with ingressing oxygen. One example of an oxidizable organic polymer is a polyether. The polyether is typically used as a polyester-ether copolymer and in low amounts of less than 10 weight percent of the packaging material. The polyester-ether is dispersed in the matrix polyester phase and interacts with a suitable oxygen scavenging catalyst that catalyzes the reaction of the ingressing oxygen with the polyether. Oxygen scavenging catalysts are typically transition metal compounds, for example an organic or inorganic salt of cobalt. Other examples include manganese, copper, chromium, zinc, iron and nickel.
Polyester containers comprising polyester-ethers and an oxygen scavenging catalyst show excellent oxygen barrier properties. However, polyethers are also lacking in stability. During preparation and processing the polyether-containing material into articles and containers, undesirable degradation products such as acetaldehyde, tetrahydrofuran, and other C2- to C4-molecules may be produced in various amounts. These side products can inter alia cause undesirable off-tastes in the product. The problem is aggravated by the presence of the transition metal oxygen scavenging catalyst. The oxygen scavenging catalyst may also catalyze polyether degradation reactions. However, the transition metal based oxygen scavenging catalyst may impart color to the resin and may catalyze unwanted degradation processes in the resin. Therefore, it is often desirable to minimize the amount of metal based oxygen scavenging catalysts.
The amount of degradation products may in turn be reduced by adding stabilizers to the resin blend. It is commonly believed that these stabilizers reduce the amount of degradation products by scavenging radicals generated during production of the resins and their processing to the final articles. However, the use of such stabilizers is considered to be problematic in its own way: Stabilizers are considered to attenuate all radical reactions. Since the oxygen scavenging reaction also involves a transition metal-catalyzed radical mechanism, the presence of such stabilizers is considered to also negatively affect the oxygen barrier properties. In other words, the use of stabilizers reduces side-products in the packaging material but also deteriorates the oxygen barrier properties. Therefore, the use of stabilizers is limited in practical application.
There is a need in the art to provide polyether-containing resins which have reduced amounts of degradation products such as acetaldehyde, tetrahydrofuran, and other C2- to C4-molecules but still provided excellent oxygen-scavenging properties.
For some applications, for example juice applications, it is advantageous that the induction time, i.e. the time lapsed until the barrier material effectively scavanges ingressing oxygen, is as short as possible. Many polyester-ether containing polyester resins provide overall excellent barrier properties but have rather long induction times. Thus, there is a need for barrier materials that have shorter induction times.
One method of addressing gas permeability involves incorporating an oxygen scavenger into the package structure itself. In such an arrangement, oxygen scavenging materials constitute at least a portion of the package, and these materials remove oxygen from the enclosed package volume which surrounds the product or which may leak into the package, thereby inhibiting spoilage and prolonging freshness in the case of food products.
Suitable oxygen scavenging materials include oxidizable organic polymers in which either the backbone or the side-chains of the polymer react with oxygen. Such oxygen scavenging materials are typically employed with a suitable catalyst, for example, an organic or inorganic salt of a transition metal such as cobalt.
One example of an oxidizable organic polymer is a polyether. The polyether is typically used as polyester-ether copolymer and in low amounts of less than 10 weight percent of the packaging material. Typically, the polyester-ether is dispersed in the polyester phase and forms discrete domains within this phase.
Polyester containers comprising polyester-ethers and an oxidation catalyst show excellent oxygen barrier properties, but suffer from a delamination phenomenon: When such containers are subjected to shock, e.g. by dropping the container from greater heights, the container may delaminate. This is a surprising type of material failure since the container is a monolayer bottle made from a homogeneous blend of polyester and polyester-ether. Nevertheless, the bottle delaminates as if it is made of a multilayer material. Delamination is a major concern for the packaging industry since delaminated containers may leak and since customers may not be willing to accept defects in appearance caused by delamination. In addition, delamination may have a negative impact on barrier properties. Also in film applications delamination may be undesirable.
There is a need in the art to provide oxygen-scavenging materials having a reduced delamination behavior.